1. Field of the Invention
This application relates generally to the field of coupling devices for electrical circuits and in particular to the field of directional couplers.
2. Relevant Background of the Invention
Directional couplers which include parallel microstrip conductors mounted on a dielectric, commonly referred to as microstrip couplers, are widely used in various types of circuits, including high frequency RF (radio frequency) and microwave circuits. Microstrip couplers are often used in connection with signal sampling (power monitoring), signal splitting and combining, signal injection and other applications.
If a directional coupler is not properly terminated, reflected waves travel back from the load to the input. These reflected waves cause degradation in the performance of the system. In a type of conventional microstrip coupler called a Lange coupler, wire or ribbon conductors are typically used to form “controlled capacitance bridges.” Controlled capacitance bridges are often used to connect alternating split microstrip conductors and these bridges typically reduce parasitic inductance. However, there is typically a parasitic capacitance associated with an controlled capacitance bridge that is not easily controlled. Such parasitic capacitance affects the circuit performance adversely. Since this capacitance affects coupler performance, it is desirable to control the amount of capacitance present and account for the amount of capacitance present while designing the coupler. The Lange coupler is described in U.S. Pat. No. 3,516,024 (“the Lange patent”), which is hereby incorporated by reference.
The characteristic impedance of a microstrip coupler is a function of the product of the impedances of the even and odd modes of TEM transmission. The degree of coupling is a function of the ratio of the even and odd mode impedances. Odd and even mode phase velocities in the microstrip conductors are not equal and this difference in velocity leads to poor directivity. The directivity generally becomes worse as the coupling is decreased. As will be appreciated by those skilled in the art, a compensating capacitor is typically placed between one or more coupled microstrip conductors and an input microstrip conductor to improve directivity.
Accordingly, port impedance, coupling, and directivity are important characteristics that need to be considered in the design of a directional coupler in order to achieve proper termination. However, in a conventional broadside-coupled directional coupler, the coupling and matching port impedance cannot be independently adjusted. As a result, circuit designers must often abandon the directional coupler approach and use alternative circuit designs, or use an additional matching circuit to complete a circuit design. Thus, it would be desirable to provide a coupler that utilizes a controlled parasitic capacitance bridge in providing a coupler having improved directivity.